My 1999 Focus 1.6 has just failed the MOT on emissions, (it barely scraped through last year) with Co reading after 2nd fast idle test of 0.7 and Hydrocarbons in excess of 300. The Lambda reading was also outside specified limits (sorry I only have the failure notice and not the emissions printout). The car runs great and has a very smooth idle. Mileage per gallon is pretty good at about 26-28mpg on average and much better on a long motorway run. I have renewed the air filter, and done an oil change with the correct synthetic 5w30 oil as part of a full service just a few weeks ago. The car does not smoke but there is a lot of soot in the exhaust tailpipe.
The guy at the garage, whom I have used for years and is trustworthy, said he suspected something sinister such as a leaking valve, worn bores etc etc, but when I removed the spark plugs to check for any mixture/ oil burning issues, the plugs were clean with only light tan/grey deposits, and this is after 12000 odd miles since they were last changed. Like I said, the idle is smooth and there are no hints of a misfire at any speed.
The only thing I can think of is that the "dealer" I got it from 18 months ago had to replace the exhuast after a few days 'cos it was blowing. I now suspect he used a secondhand exhaust and possibly also a second hand Cat. Is it possible that the cat was coked up and the high emissions reading is from deposits within it being burnt by the hot exhaust gasses? as the clean spark plugs don't indicate and bore/valve/mixture issues. MOT runs out on wednesday so any ideas anyone?